1. Field of the Invention
The present invention relates to injection molding machines and die closing units for injection molding machines, and, more particularly, to a method and apparatus for the injection of confluent streams of different plastic materials into an injection molding die, for the production of plastic articles of marbled or mottled appearance.
2. Description of the Prior Art
In U.S. Pat. No. 2,418,856 is described a method and apparatus for injection molding by which plastic raw materials of different characteristics and colors may be blended during injection, so as to produce a mottling of the materials in the articles. This effect is obtained by means of a special injection unit which has two parallel injection cylinders with cooperating injection pistons which may be advanced in unison or independently, for the injection of two separate streams of plastic material discharging in concentric streams from the mouth of an injection nozzle, directly into the sprue channel of the injection molding die. It is inside the sprue channel and inside the mold cavity itself, where the two streams of different plastic materials come in contact with each other and are intermingled to a greater or lesser degree, depending on the path of the materials.
Another method and device for the intermingling of streams of plastic raw material, and for the subsequent injection thereof into an injection molding die, is disclosed in the German Offenlegungsschrift (Published Application) No. 24 45 107. There, the material streams from two injection units are fed through parallel channels to a revolving mixing valve which alternatingly connects one or the other of the material streams to a common central injection channel of the injection unit. This method and apparatus subdivide the material streams into a succession of separate quantitative increments or charges which enter the injection molding die in a succession of charges coming alternatingly from the two material streams. In a modified version of this prior art method and apparatus, one material stream is maintained in a continuous fashion, while the other material stream is subdivided into a succession of charges which are injected into the continuous material stream.